Two common problems which accompany the transmission and reception of amplitude modulation (AM) signals are adjacent-channel interference and envelope distortion in null regions of the radiation pattern of an AM transmitter's antenna. These and other problems are alleviated by asymmetrical sideband transmission, which is the subject matter of my prior U.S. Pat. No. 4,569,073.
As shown in my prior patent, one method for implementing asymmetrical sideband AM broadcasting is by using an independent sideband (ISB) AM stereo exciter, which inherently permits independent control of the upper and lower sidebands of an AM signal, preceded by a specialized audio signal processor that determines the relative amplitude and frequency characteristics of the two sets of sidebands in the resulting transmitted signal.
However, asymmetrical sideband transmission in accordance with my prior patent was less suitable for the transmission of stereo program material in that the intentional asymmetry introduced into the sidebands of the transmitted signal also inherently introduced a corresponding asymmetry into the transmitted and received stereo image. For example, if the lower sideband of a transmitted ISB AM stereo signal is enhanced relative to the upper sideband in order to obtain the benefits of asymmetrical sideband transmission, the location of sound sources in the resulting sound image produced by an ISB AM stereo receiver will be correspondingly shifted to the left. This occurs because in the ISB system, the lower sideband carries primarily left (L) stereo information, whereas the upper sideband carries primarily right (R) stereo information.
Because of this, my prior patent proposed that asymmetrical sideband AM transmission be used primarily for broadcasting monophonic program material, and that for stereo program material the transmitter should be switched to normal ISB AM stereo transmission. (See FIG. 3 of that patent).
It is, therefore, an object of the present invention to provide an asymmetrical sideband AM transmission system which not only alleviates the problems of adjacent channel interference and envelope distortion in null regions of a directional antenna's radiation pattern, reduces the effects of co-channel interference, reduces distortion caused by selective fading, eases the difficulty in tuning continuous tuned receivers and improves the fidelity of such receivers when side tunes, but also enables stereo program material to be transmitted and received with pleasing stereo effects on ISB AM stereo receivers.